In a known form of circuit switched TDM multiplex a particular circuit is identified from all others by imposing a frame and counting the slots from the start of the frame. This implies that the circuit rate is a fixed and exact proportion of the bearer rate; therefore the system is synchronous. In such systems small differencesin rate (plesiochronous operation) can be handled by either occasionally deleting or repeating a sample carried in a slot or by providing a bearer nominally above the synchronous rate and padding the frame to fit.
In a known asynchronous time division network discussed in U.S. Pat. No. 4,491,945 a technique of rotating the destination address is used, so that the two most significant bits of the address are always presented to the next succeeding switch node.
For an asynchronous time division network there is no frame and each packet carries its own identification and, provided the bearer rate is sufficient to give a very low probability of packets being lost due to queue overflow, there need not be a synchronous relationship.
In this type of network modest sized switch elements can be used, for example 8.times.8, but a disadvantage is that queues are needed at each switch element input port.
The network carries packets for two basic types of virtual circuit. There are STREAM circuits equivalent to a conventional circuit switched service, and BURST circuit carrying bursty data. In the latter case it may be assumed that an embedded protocol is provided to acknowledge `data packets` (corresponding to a contiguous group of switch packets) and providing a repeat facility if `data packets` are incorrectly received. Also, in the case, relatively long queueing delays are acceptable. If the bearers and ports are loaded with up to, say, 55% of STREAM traffic and STREAM packets are given first priority of access then BURST services can be offered access using the remaining capacity. In this way much higher loadings may be achieved, at least on the output ports. Providing first priority access to STREAM services in a single plane switch will give some improvement; however, if a single queue is used at the input ports and the packet at the head of the queue is a BURST packet, following STREAM packets will be blocked. A greater improvement is achieved by providing two input queues, one for STREAM packets (with first priority access) and the other for BURST packets. The BURST packet queue can also be made longer to allow for transient conditions. For a multi-stage switch a further modest improvement can be achieved by routing STREAM and BURST data through separate switch planes. This improvement results because the loading on inter-stage bearers is substantially reduced. Almost all the loss then occurs at the access point to the common output port bearer. In this case also the loss on STREAM services is negligibly small, almost all loss occurs on BURST packets.
The above solution suffers from the disadvantage that, when more than one input port wishes to send a packet to the same output port at the same time only one can succeed, thus packets further back in the queue may be blocked from free output ports. This problem can be resolved by providing a very short queue at each crosspoint of the switch matrix. Packets are then loaded from the input queue into the relevant crosspoint queue and output bearers will normally have one or more packets waiting to be transmitted on that bearer. The combination of a two plane switch with two packet crosspoint queue permits loadings of more than 80% assuming 70% STREAM and 30% BURST traffic.
A queueing algorithm is therefore required to take note of packets further back from the head of the queue. The further back the algorithm looks, the higher the achievable occupancy of the destination bearer.